For the purposes of this specification, a screwdriver is a manually-operable tool having a shaft, a handle at one end of the shaft, and a fastener-engaging bit at the other end of the shaft (the fastener-engaging bit possibly being substituted by a plug or a socket (or other coupling) for the temporary attachment of a detachable fastener-engaging bit of a selected size and/or shape). The fastener to be operated by the screwdriver may be a screw, a bolt, a nut or some other form of fastener, such fasteners having the common characteristic that they are all operated by rotation (either for tightening or loosening), the requisite torque being delivered to the fastener through the shaft of the screwdriver. More effective use of such fasteners may be achieved if the torque applied by the screwdriver to the fastener is known with a reasonable degree of precision, as distinct from the relatively haphazard estimation by the user of manual effort being applied through the screwdriver.